The present invention relates to an apparatus and method for projecting an alignment image. More particularly, the present invention relates to an apparatus and method that generates a projected reticle image to facilitate the calibration between a moveable direct probe sensor camera and a fixed camera.
Improvements in manufacturing processes has led to an increase in the density and complexity of semiconductor devices placed on a single silicon wafer. The increased density of semiconductor devices, however, has reduced the accuracy of wafer sorts. Wafer sort, or wafer probe, describes the process of using probe cards to identify semiconductor devices at the wafer stage of manufacture that have inter-connectivity or electrical malfunctions prior to the individual packaging of the semiconductor devices. In particular, a probe card includes a collection of electrical contacts, pins, or probes that are positioned to make contact with the bonding pads of the semiconductor device under test (xe2x80x9cDUTxe2x80x9d). Subsequently, Automatic Test Equipment (xe2x80x9cATExe2x80x9d) electrically connected to the probe cards, generates electrical tests to examine the inter-connectivity or electrical operation of the DUT.
As the density of semiconductor devices increase, the dimensions of the probe card have dramatically shrunk to ensure proper probe-to-pad alignment. Probe-to-pad alignment describes accurately positioning the bonding pads of a semiconductor device located on a wafer in such a way that the bonding pads of the device make good electrical contact with the probe tips of the probe card. The modified probe card dimensions, however, create numerous problems during probe-to-pad alignment. To ensure accurate probe-to-pad alignment numerous methods have been developed in the prior art.
One method of a prior art probe-to-pad alignment uses a dummy wafer in conjunction with an auto-align fixed camera. The fixed camera is a downward looking camera with a fixed position and a known field of view. Using the fixed downward looking camera to view the bonding pads and other features on a wafer, the location of the bond pads on the DUT are determined in horizontal dimensions xe2x80x98xxe2x80x99 and xe2x80x98yxe2x80x99. The xe2x80x98zxe2x80x99, or vertical location, of the wafer surface, or equivalently, of the bond pads, is determined using a separate system. Next, a dummy wafer with a soft markable surface, such as an aluminum layer, is probed. The probing causes the probe tips to leave indentations on the dummy wafer. Based on the location of the probe indentations the fixed camera determines the xe2x80x98x-yxe2x80x99 coordinates of the probe tips relative to the dummy wafer. Using the derived xe2x80x98x-yxe2x80x99 coordinates of the probe tips, the prober positions the bond pads of a DUT in contact with the probe tips. Thus, probe-to-pad alignment is achieved. The method of using dummy wafers for probe-to-pad alignment, however, has numerous drawbacks. In particular, this method results in wasted wafers, possible damage of probe tips, reliance on an alternate system to measure xe2x80x98zxe2x80x99 coordinates, and reliance on probe indentations to interpret actual probe tip position.
To counteract the reliance on dummy wafers, prior art probers developed a direct probe sensor (xe2x80x9cDPSxe2x80x9d) camera. In the prior art, the DPS camera is used in conjunction with the fixed camera to align probe tips and bond pads. In particular, the DPS camera is an upward looking camera that records the x, y, and z coordinates of the probe tips of a probe card. As previously described, the fixed camera is a down ward looking camera that determines the x, y, and z coordinates of the bond pads of a DUT located on a wafer. Based on the x, y, and z coordinates of the probe tips and the bond pads, the prober positions the wafer to align the probe tips of the probe card with the bond pads of the DUT.
FIG. 1 illustrates a prior art prober using a DPS camera. In particular, system 100 includes a probe card 160 with probe tips 165. System 100 also includes lens system 120, physical reticle 140, and DPS 110xe2x80x94a charge coupled device (xe2x80x9cCCDxe2x80x9d) that records images on pixel grid 115. System 100 records the location of probe tips 165 via lens system 120. System 100 also includes wafer chuck 170. Wafer chuck 170 is coupled to lens system 120. System 100 moves wafer chuck 170 in the x, y, and z coordinates to place a wafer (not shown) in contact with probe tips 165. System 100 also moves wafer chuck 170 in the x, y, and z coordinates to record the location of probe tips 165.
Prior to recording the probe tip locations, the x, y, and z coordinates of the field of view of DPS 110 is calibrated with a fixed camera (not shown). As previously described, the fixed camera is a downward looking camera with a fixed position and a known field of view. The calibration between DPS 110 and the fixed camera is performed via physical reticle 140. In the prior art, physical reticle 140 is a thin plate of glass with cross-hair pattern 150 located in the center of the glass plate. During calibration, physical reticle 140 is placed at the focal point of DPS 110xe2x80x94denoted as focal 180. Using the image generated by cross-hair pattern 150, DPS 110 generates a pixel representation of cross-hair pattern 150 on pixel grid 115. The pixel representation is relayed to a prober (not shown). Subsequently, housing 170 moves physical reticle 140 under the fixed camera and the fixed camera""s field of vision relative to cross-hair pattern 150 is determined and relayed to the prober.
The prober correlates the pixel representation of cross-hair pattern 150 generated by DPS 110 to the known location and field of view of the fixed camera. Thus, the position of a probe tip viewed by DPS 110 is accurately determined because both cameras, DPS 110 and the fixed camera, are calibrated to each other by focusing on the same intermediate targetxe2x80x94cross-hair 150. Using physical reticle 140 for alignment between DPS 110 and the fixed camera, however, create numerous disadvantages.
One disadvantage of using a physical reticle results from the design characteristics of the physical reticle. In particular, as previously described, physical reticle 140 is designed using a glass plate. The glass pate, however, creates an image offset because there is an optical path difference between glass and the air surrounding physical reticle 140. The image offset results in a shifted cross-hair 150, which in turn results in a calibration offset in the xe2x80x9czxe2x80x9d direction.
Another disadvantage of using a physical reticle results from the requirement of operator intervention of the physical reticle. In particular, physical reticle 140 is removed during non-calibration (i.e. normal testing) use. Thus, full automation is prevented.
Yet another disadvantage of using a physical reticle results from the close proximity of the physical reticle to the probe tips. In particular, during the calibration of DPS 110, the physical reticle 110 may cause damage to the probe tips through accidental contact.
A testing system operable to accurately position a plurality of contact electrodes relative to a plurality of electrical contacts is disclosed. For one embodiment, the testing system comprises a first imaging system coupled to a wafer chuck. The wafer chuck is used to place the electrical contacts of a wafer in contact with the plurality of electrodes. To facilitate accurate positioning between the wafer electrical contacts and the contact electrodes, the first imaging system is configured to locate the plurality of contact electrodes. The testing system also comprises a second imaging system configured to locate the wafer electrical contacts. To calibrate the objects viewed by the first imaging system and the second imaging system, an image generator coupled to at least one of the imaging systems generates an alignment image along the optical path of the imaging system. The testing system calibrates positioning and imaging information between the first imaging system and the second imaging system using the alignment image.
According to another embodiment, an imaging system operable to generate an alignment image is disclosed. The imaging system comprises an image generator configured to generate the alignment image. The imaging system also comprises an objective coupled to the image generator that has an optical path including an objective lens, a rear image forming lens, and a beam-splitter coupled between the objective lens and the rear image forming lens. The beam-splitter is configured to inject the alignment image into the optical path of the imaging system. For one embodiment, the imaging system generates the alignment image on the focal point of the imaging system via a charge coupled device. Specifically, a reflective charge coupled device is coupled to the objective. The reflective charge coupled device is configured to reflect the alignment image onto the focal point of the imaging system.
For yet another embodiment, the alignment image projected on the charge coupled device and the reflected alignment image are optically conjugate points. Thus, a second imaging system viewing the projected alignment image of a first imaging system results in both imaging system viewing the identical image at the same point in space.